The only singlet oxygen oxidation (1O2-Ox) which is currently carried out industrially is the photochemical 1O2-Ox in which the 1O2 is generated by a photochemical route. The disadvantage of this process is given by the high costs of the photochemical equipment required, and by a limited service life. The required lamps degenerate relatively rapidly during the oxidation as a result of soiling of the glass surface. In addition, this process is not suitable for colored substrates. The process is actually suitable only for fine chemicals which are prepared on a relatively small scale. (La Chimica e l""Industria, 1982, Vol. 64, page 156).
For this reason, attempts have been made to find other process variants for the 1O2-Ox which are suitable for the 1O2-Ox of non-water-soluble, hydrophobic organic substrates.
J. Am. Chem. Soc., 1968, 90, 975 describes, for example, the classical xe2x80x9cdarkxe2x80x9d 1O2-Ox in which 1O2 is not generated photochemically, but chemically. In this process, hydrophobic substrates are oxidized by means of a hypochlorite/H2O2 system in a solvent mixture of water and organic solvent. However, this process has only found a few synthetic applications since many substrates are only sparingly soluble in the required medium. In addition, the use possibility is rather limited because of secondary reactions between hypochlorite and substrate or solvent. In addition, a large part of the 1O2is deactivated in the gas phase. In addition, this process is not suitable for industrial scale since in the organic medium addition of the hypochlorite on H2O2 results, and a large excess of H2O2 is required to suppress the secondary reaction of substrate with hypochlorite. An additional disadvantage arises as a result of the formation of stoichiometric amounts of salt.
A variant of the xe2x80x9cdarkxe2x80x9d 1O2-Ox, which is not based on hypochlorite and thus should partly avoid the above disadvantages, is known, for example, from J. Org. Chem., 1989, 54, 726 or J. Mol. Cat., 1997, 117, 439, according to which some water-soluble organic substrates are oxidized with H2O2 and a molybdate catalyst in water as solvent. According to Membrane Lipid Oxid. Vol. II, 1991, 65, the 1O2-Ox of water-insoluble, organic substrates with the molybdate/H2O2 system is difficult since it was assumed that none of the customary solvents is suitable for maintaining the disproportionation, catalyzed by molybdate, of H2O2 into water and 1O2. However, the use of molybdenum catalysts also brings other disadvantages with it. For example, in addition to the H2O2 disproportionation, they also catalyze other undesired oxidations of some substrates. For example, allyl alcohols cannot be effectively peroxidized with the molybdate/H2O2 system since this group of substances is epoxidized by molybdenum in the presence of H2O2. A further disadvantage of these catalysts is the relatively low pH range in which they function. These catalysts can only be used in the basic range between pH9 and pH 12, the use of this system is accordingly unsuitable for acidic conditions.
Accordingly, it was an object of the present invention to catalysts for the H2O2 disproportionation for xe2x80x9cdarkxe2x80x9d 1O2-Ox which are effective in a broad pH range, in particular including in the acidic range and which do not catalyze undesired secondary reactions, such as, for example, the epoxidation of allylic alcohols, in addition to the H2O2 disproportionation.
Unexpectedly, it has now been found that lanthanides are effective as catalysts both in the basic and also in the acidic range, with undesired secondary reactions not arising or arising only to a considerably lesser degree when said lanthanides are used. Unexpectedly, these catalysts are also active in heterogeneous form, meaning that their recovery from the reaction mixture can be carried out in a simple way.
Accordingly, the present invention provides a process for the oxidation of organic substrates by means of 1O2, which comprises adding 30-70% strength H2O2 to organic substrates which react with 1O2 in water, in an organic solvent or in water/solvent mixtures in the presence of a lanthanide as catalyst, whereupon, following the catalytic decomposition of H2O2 to give water and 1O2, oxidation to give the corresponding oxidation products takes place.
The process according to the invention is suitable for the oxidation of organic substrates which react with 1O2.
Accordingly, substrates which may be used are the following compounds: olefins which contain one or more, i.e. up to 10, preferably up to 6, particularly preferably up to 4 Cxe2x95x90C double bonds; electron-rich aromatics, such as C6-C50, preferably up to C30, particularly preferably up to C20, phenols, polyalkylbenzenes, polyalkoxybenzenes; polycyclic aromatics having 2 to 10, preferably up to 6, particularly preferably up to 4 aromatic rings; sulfides, such as, for example, alkyl sulfides, alkenyl sulfides, aryl sulfides which are either mono- or disubstituted on the sulfur atom, and heterocycles having an O, N or S atom in the ring, such as, for example, C4-C50, preferably up to C30, particularly preferably up to C20, furans, C4-C50, preferably up to C30, particularly preferably up to C20, pyrroles, C4-C60, preferably up to C30, particularly preferably up to C20, thiophenes. In this connection, the substrates may have one or more substituents, such as halogen (F, Cl, Br, I), cyanide, carbonyl groups, hydroxyl groups, C1-C50, preferably up to C30, particularly preferably up to C20, alkoxy groups, C1-C50, preferably up to C30, particularly preferably up to C20, alkyl groups, C6-C50, preferably up to C30, particularly preferably up to C20, aryl groups, C2-C50, preferably up to C30, particularly preferably up to C20, alkenyl groups, C2-C50, preferably up to C30, particularly preferably up to C20, alkynyl groups, carboxylic acid groups, ester groups, amide groups, amino groups, nitro groups, silyl groups, silyloxy groups, sulfone groups, sulfoxide groups. In addition, the substrates may be substituted by one or more NR1R2 radicals in which R1 or R2 may be identical or different and are H; C1-C50, preferably up to C30, particularly preferably up to C20, alkyl; formyl; C2-C50, preferably up to C30, particularly preferably up to C20, acyl; C7-C50, preferably up to C30, particularly preferably up to C20, benzyl, where R1 and R2 may also together form a ring, such as, for example, in a phthalimido group.
Examples of suitable substrates are: 2-butene; isobutene; 2-methyl-1-butene; 2-hexene; 1,3-butadiene; 2,3-dimethylbutene; xcex949,10-octalin, 2-phthalimido-4-methyl-3-pentene; 2,3-dimethyl-1,3-butadiene; 2,4-hexadiene; 3-methyl-2-buten-1-ol; 4-methyl-3-penten-2-ol; 2-amino-4-methyl-3-pentene; 2-chloro-4-methyl-3-pentene; 2-bromo-4-methyl-3-pentene; 1-trimethylsilylcyclohexene; 2,3-dimethyl-2-butenyl-para-tolylsulfone; 2,3-dimethyl-2-butenyl-para-tolyl sulfoxide; N-cyclohexenylmorpholine; 2-methyl-2-norbornene; terpinolene; xcex1-pinene; xcex2-pinene; xcex2-citronellol; ocimene; citronellol; geraniol; farnesol; terpinene; limonene; trans-2,3-dimethyl-acrylic acid; xcex1-terpinene; isoprene; cyclopentadiene; 1,4-diphenylbutadiene; 2-ethoxybutadiene; 1,1xe2x80x2-dicyclohexenyl; cholesterol; ergosterol acetate; 5-chloro-1,3-cyclohexadiene; 3-methyl-2-buten-1-ol; 3,5,5-trimethylcyclohex-2-en-1-ol; phenol, 1,2,4-trimethoxybenzene, 2,3,6-trimethylphenol, 2,4,6-trimethylphenol, 1,4-dimethylnaphthalene, furan, furfuryl alcohol, furfural, 2,5-dimethylfuran, isobenzofuran, dibenzyl sulfide, (2-methyl-5-tert-butyl)phenyl sulfide etc.
As a result of the oxidation according to the invention, the corresponding oxidation product is obtained from the substrates. Alkenes, (polycyclic) aromatics or heteroaromatics give, in particular, hydroperoxides or peroxides which are able to further react under the reaction conditions to give alcohols, epoxides, acetals or carbonyl compounds, such as ketones, aldehydes, carboxylic acids or esters, if the hydroperoxide or the peroxide is unstable.
The oxidation according to the invention is carried out in water or an organic solvent.
Accordingly, suitable solvents are water, C1-C8-alcohols, such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol; ethylene glycol, propylene glycol, formamide, N-methylformamide, dimethylformamide, sulfolane, propylene carbonate.
Preference is given to using methanol, ethanol, propanol, isopropanol, ethylene glycol, propylene glycol, formamide, N-methylformamide or dimethylformamide, particularly preferably methanol, ethanol, ethylene glycol, propylene glycol, formamide or dimethylformamide as solvent.
The reaction can also be carried out in a water/solvent mixture or in a mixture of the above organic solvents.
A metal, namely a lanthanide, is added to the solvent/substrate mixture as heterogeneous or homogeneous inorganic catalyst. Suitable lanthanides are lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium and lutetium, and the so-called pseudo-lanthanides scandium and yttrium. Preference is given to using lanthanum.
In this connection, the metal can be in forms customary for 1O2 oxidations, for example as the oxide, oxo complex, nitrate, carboxylate, hydroxide, carbonate, chloride, fluoride, sulfate, tetrafluoroborate, etc. Where appropriate, a hydroxide, for example NaOH, KOH, etc., can be added to homogeneous, soluble forms of the catalyst to give a heterogeneous, active catalyst.
The amount of catalyst used depends on the substrate used and is between 1 and 15 mol %, preferably between 5 and 25 mol %.
This is followed by the addition of 30-70% strength, preferably 40-60% strength, H2O2. H2O2 is preferably added slowly or in portions to the reaction mixture of solvent, substrate and catalyst, the reaction mixture being stirred. It is also possible to firstly add only some of the H2O2 to a soluble form of the chosen catalyst, then to add a hydroxide, such as, for example, NaOH, KOH etc., and thereafter the residual amounts of H2O2.
In the process according to the invention, the consumption of H2O2 is dependent on the substrate used. For reactive substrates, 2 to 3 equivalents of H2O2 are preferably needed, while less reactive substrates are preferably reacted with 3 to 10 equivalents of H2O2.
The reaction temperature is between 0 and 50xc2x0 C., preferably between 15 and 35xc2x0 C. If the reaction is carried out in water, then the pH of the reaction mixture depends on the substrate chosen. The pH is between 0 and 14, preferably between 4 and 14. The pH of the reaction mixture can be adjusted where necessary as required using customary basic or acidic additives.
The course of the reaction can be monitored by means of UV spectroscopy or by means of HPLC. When the reaction is complete, i.e. after 1 to 30 hours, depending on the reaction conditions, the reaction mixture is worked up and the catalyst is separated off by customary methods.
Particularly in cases where a heterogeneous catalyst is used, separating off the catalyst is very readily possible by simple filtration OR CENTRIFUGATION, as a result of which recycling of the catalyst is also simplified.
The end-product which remains can, where appropriate, be purified by means of recrystallization, extraction or distillation.
The process according to the invention permits the oxidation of a large number of compounds. Accordingly, the process according to the invention is particularly suitable for the oxidation of unsaturated organic compounds, such as allyl alcohols, unsaturated amines, such as allylamines, terpenes, for example xcex1-terpinene and citronellol, aromatic polycycles, steroids, furans, cyclopentadienes, phenols etc., and generally for all compounds which react with 1O2.
The process according to the invention gives the desired end-products in high yields of up to 100% with high purity.
The process according to the invention is characterized by the simple process regime which is best suited to the industrial scale since it can take place in simple multipurpose plants and with simple work-up steps, and can be used for a wide spectrum of substrates.